ofp-util: Use correct cookie value in "packet_in"s when no flow involved.

ofp-util: Use correct cookie value in "packet_in"s when no flow involved.

OpenFlow 1.3 uses all-1-bits in a packet_in to indicate that the packet_in
was not generated by a flow, but Open vSwitch incorrectly used 0.  This
fixes the problem.

For consistency, this commit also changes NXT_PACKET_IN to use all-1-bits
for this case, event though NXT_PACKET_IN was previously defined to use
zero.  This doesn't appear to make a difference for the NVP controller; if
it causes a problem for some other controller then I will revert that part
of the change.

Found by inspection.

Signed-off-by: Ben Pfaff <blp@nicira.com>